Conventional gravimetric weighing instruments include weighing cells based on a variety of different operating principles such as for example weighing cells with strain gauges, weighing cells with string oscillators, or electromagnetic force-compensation (EMFC) weighing cells. Gravimetric measuring instruments with string oscillator weighing cells or electromagnetic force-compensation weighing cells deliver measurement results with a very high resolution.
In EMFC weighing cells, the weight of the load is transmitted—either directly or through one or more force-transmitting levers—to a measurement transducer which produces an electrical signal corresponding to the weighing load. By means of a weighing-oriented electronic module, the electrical signal is further processed and displayed on an indicator.
The mechanical design arrangement of weighing cells with string oscillators is largely analogous to EMFC weighing cells, with the difference that an oscillating string transducer is used instead of an electromagnetic measurement transducer. The load translates into an amount of tension of the oscillating string. The resultant frequency change of the string's oscillation, in turn, represents a measure for the load placed on the weighing pan. At the point in time when the measurement value is being captured, the mechanical system of an EMFC weighing cell is in a state of equilibrium similar to a mechanical beam balance with counterweights. In contrast, the load-receiving part of a string oscillator weighing cell undergoes a small vertical displacement in relation to the stationary part, as the string is set under tension by the load, whereby its length is slightly increased. String oscillator weighing cells are therefore also referred to as small-displacement force-measuring cells.
Both types of weighing cells are used for example in precision balances and analytical balances in the milligram range or in microbalances in the microgram range and need to be periodically recalibrated in order to ensure that the measurement values delivered by them lie within a prescribed tolerance range in accordance with the manufacturer's specifications and legal requirements. These periodic calibrations serve to compensate for the influence of factors that affect the weighing cell, for example changes of the ambient temperature or barometric pressure.
The calibration is made by periodically setting a load of known weight on the load-receiving part. Based on the difference between the weight value that was established in the final test prior to delivery of the weighing cell and the currently measured value, a correction value can be calculated by means of which the subsequent measurement results of the weighing cell can be corrected. In order to obtain the most accurate calibration value possible, the calibration weight should equal the maximum capacity load of the weighing cell. This can mean that very large calibration weights will be necessary.
Among the prior art, a variety of different gravimetric measuring instruments are known which include a built-in calibration weight.
A gravimetric measuring instrument of this kind which is based on the principle of electromagnetic force compensation and has a built-in rod-shaped calibration weight is disclosed in EP 0 955 530 B1. The rod-shaped calibration weight is set on a calibration weight carrier arm which is coupled to the load-receiving part and serves as a ratio lever. Due to this lever advantage, the mass of the calibration weight, and thus its dimensions, can be kept small. As the calibration weight arm is always coupled to the load-receiving part, it serves only for the purposes of receiving and leveraging the calibration weight during the calibration process, but is not part of the calibration weight itself. Consequently, the calibration weight carrier arm is part of a force-transmitting device, more specifically of a lever mechanism for transmitting and reducing the force before it reaches the measurement transducer, and remains permanently connected to the load-receiving part also while the balance operates in normal weighing mode.
As disclosed in CH 661 121 A5, the force-transmitting device can also include a multi-stage lever mechanism, wherein the individual levers are suitably connected to each other by means of coupling elements, so that a force reduction is achieved between the load-receiving part and the measurement transducer. Formed on one of the coupling elements are suitably designed receiving elements on which to set a calibration weight.
In JP 3761792 B2 a weighing cell equipped with strain gauges is disclosed which has a calibration weight with a ratio lever. A coupling element is arranged between the ratio lever and the load-receiving part. By raising the calibration weight and the coupling element, a bearing block which is formed on the coupling element is separated from a knife edge pivot which arranged on the load-receiving part, whereby the ratio lever is uncoupled from the load-receiving part.
All of the foregoing conventional solutions are equipped with calibration-weight-loading devices that are familiar to professionals in the weighing equipment field.
The precise determination of the correction value depends not only on the resolution capabilities of the measurement transducer but also to a substantial degree on the level of precision at which the geometric relationships are maintained. Even the smallest deviations in the seating position of the calibration weight from its specified position on the calibration weight carrier arm described in EP 0 955 530 B1, or on the coupling member described in CH 661 121 A5, or the smallest shifts in the position of the bearing block relative to the knife edge pivot in JP 3761792 B2, will cause a lengthening or shortening of the effective lever arm and thus to an error in the correction value. Consequently, the points of contact between the calibration weight and the calibration weight carrier arm, or between the knife edge pivot and the bearing block, are finished with the highest precision at a correspondingly high cost.